1. Field of the Invention
The present invention relates to a method for manufacturing a filter, and more particularly, to a method for manufacturing a microwave bandpass filter including a plurality of microstrip resonators arranged in series.
2. Description of the Related Art
Generally, in communication apparatuses, microwave filters have been developed. A typical microwave filter is constructed by a dielectric substrate, a ground plane formed on a back surface of the dielectric substrate, and a plurality of microstrip conductors arranged in series on a front surface of the dielectric substrate, so that a plurality of microstrip resonators are formed in series.
In a prior art method for manufacturing a filter, basic parameters such as a shape of the microstrip resonators, the number of the microstrip resonators and the like are determined. Then, coupling coefficients are calculated to satisfy desired frequency responses of the filter. Then, the coupling coefficients are converted into distances, in accordance with an experimental relationship between a distance between two resonators and a coupling coefficient between the two resonators. Then, a layout for the filter is designed by using the basic parameters and the obtained distances, and the filter is manufactured in accordance with the layout. Then, it is determined whether or not the frequency responses of the filter satisfy the desired frequency responses.
When the frequency responses of the filter do not satisfy the desired frequency responses, a laser trimming operation is performed upon some of the resonators, or tuning screws provided above the resonators are adjusted, thus repeating the determination of the frequency responses of the filter. This will be explained later in detail.
In the above-described prior art manufacturing method, however, since the frequency responses or the filter after the adjustment are non-linear, it is difficult for the obtained frequency responses of the filter to be converged into the desired frequency responses. At worst, the obtained frequency responses are further deviated from the desired frequency responses, which may make it necessary to redetermine the basic parameters.
Also, the above-mentioned laser trimming operation or the adjustment of tuning screws needs to be performed upon each manufactured filter, which would remarkably increase the manufacturing cost.
Thus, the manufacturing cost is increased.
It is an object of the present invention to provide a method for manufacturing a filter capable of decreasing the manufacturing cost.
According to the present invention, in a method for manufacturing a filter including a plurality of resonator arranged in series, a shape of the resonators and a number of the resonators are determined, so that an amount of additional coupling coefficients between the resonators except for two adjacent ones of the resonators is smaller than a predetermined value. Then, an initial coupling coefficient function is calculated with respect to a distance between two adjacent ones of the resonators, and is set in a coupling coefficient function. Then, coupling coefficients between the resonators are calculated for desired filter frequency responses, and distances between the resonators having the calculated coupling coefficients are calculated in accordance with the coupling coefficient function. Then, a layout of the filter having the resonators with the calculated distances is laid out and a tentative filter is manufactured. Then, it is determined whether or not filter frequency responses of the tentative filter satisfy the desired filter frequency responses. Only when the filter frequency responses of the tentative filter satisfy the desired filter frequency responses, are actual filters manufactured by using the above-mentioned layout. Otherwise, the coupling coefficient function is changed, thus repeating the distance calculating step, the layout designing step and the filter frequency responses determining step.
Since the coupling coefficient function is changed step by step, the frequency responses of the filter can be easily converged into the desired frequency responses.